As part of the standard processing of CMOS integrated circuits and semiconductor devices, it is desirable to smooth the underlying topography following gate definition to ensure smooth and continuous metal step coverage throughout the circuit. Typically, a bilayer sandwich, i.e. a top layer of phosphosilicate glass (PSG) and a bottom layer of undoped silicon dioxide, is deposited on the circuit to provide passivation, gettering, and electrical insulation between metal interconnects and underlying circuit structures. Following deposition, the PSG layer is reflowed and densified in a furnace. This smooths the surface topography to ensure continuity of the overlying metallization. The undoped oxide, which does not undergo appreciable reflow, prevents counterdoping of active device regions during high temperature processing.
Following furnace reflow, it is necessary to open windows in the oxide sandwich to permit electrical contact to the elements (i.e. source and drain regions) of each transistor of the integrated circuit. Acceptable metal step coverage throughout the windows normally requires that the contact opening etch produce sloped or tapered sidewalls. The present invention relates to an improved method for wet etching the windows in the oxide layer to achieve tapered contact sidewalls.